US 7208402 B2
An apparatus comprising: a die having a top metal layer, the top metal layer comprised of at least a first metal line and a second metal line; a passivation layer covering the top metal layer; a C4 bump on the passivation layer; and a first passivation opening and a second passivation opening in the passivation layer, the first passivation opening to connect the first metal line to the C4 bump, and the second passivation opening to connect the second metal line to the C4 bump.
1. A method comprising:
forming a passivation layer over the top surface of a semiconductor device, the top surface of the semiconductor device containing a plurality of metal lines wherein said plurality of metal lines are each connected to a lower metal layer beneath said plurality of metal lines by a via;
forming a plurality of passivation openings in the passivation layer to expose portions of each of the plurality of metal lines; and
placing a bump on the passivation layer such that the bump is placed over at least two of the plurality of passivation openings and is connected to at least two of the plurality of metal lines.
2. The method of
3. The method of
4. The method of
5. The method of
6. The method of
7. The method of
8. The method of
9. The method of
10. The method of
11. The method of
This is a Divisional application of Ser. No. 10/739,726 filed Dec. 17, 2003, which is presently pending.
1. Field of the Invention
The present invention relates generally to the field of semiconductor packaging, and more specifically to the interconnect between a C4 bump and a semiconductor die.
2. Discussion of Related Art
Presently, a semiconductor die is connected to a flip-chip package by a two dimensional array of C4 bumps. Power is routed through the package to the die via these C4 bumps. Power is further distributed from these C4 bumps to different parts of the die through metal interconnect lines in the top metal layers of the die.
Power is routed through the die by way of a number of metal layers. The top two metal layers are illustrated in
Current density and the ability for the power grid to reliably deliver current is a function of the metal stack and the EM (electromigration) capabilities of the metals and vias in the metal stack. Presently EM issues may be solved in several ways. For example, for a single passivation opening connection, when current crowding exceeds EM margins, an additional metal layer may be added to the metal stack to reduce current crowding, however this may increase the cost of manufacturing. Another alternative that may be used to reduce current crowding is to use a thicker metal layer or increase the pitch of the metal lines. This alternative may result in an undesirable reduction of signal routing capability. A further option is doping of the metal layers to allow for greater EM margins. Doping of the metal layers may increase the resistance of the metal lines, which is undesirable.
In the following description, numerous specific details are set forth, such as exact process steps, in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known components or methods have not been described in detail in order to avoid unnecessarily obscuring the present invention.
A method of connecting a single C4 bump to multiple metal lines on the top metal layer of a die through multiple passivation openings is described, as well as methods of forming this connection. The use of multiple passivation openings to connect a single C4 bump may decrease electrical resistance between the die and the package. Current crowding and IR (voltage) drop on lower metal layers may also be reduced through the use of multiple passivation openings. An additional metal layer may not be required to reduce current crowding or IR drop.
The use of multiple passivation openings to connect the C4 bump to multiple metal lines may increase the total connection area between the C4 bump and the metal lines, as compared to the use of a single passivation opening and a single metal line. This increase in total connection area may effectively decrease the electrical resistance between the package and the die.
Each of the passivation openings 206 may be the same size, or they may be of different sizes. While the passivation openings 206 are shown vertically oriented with respect to each other in
A passivation layer 210 formed on the top surface of the die 200 protects the surface of the die from external contamination and dust. Openings 206 are formed in the passivation layer such that the C4 bump 202 may be connected to multiple top layer metal lines 204. In one embodiment of the present invention, a C4 bump may be positioned over two passivation openings so that it may connect to two metal lines. In other embodiments, the C4 bump may be connected to more than two metal lines through more than two passivation openings. At least one passivation opening may be provided to connect the C4 bump to each metal line. In another embodiment, multiple passivation openings may be provided to connect the C4 bump to each metal line. In one embodiment, ball limiting metallurgy (BLM) 215 may be present beneath the C4 bump. The BLM layer(s) may be comprised of materials including, but not limited to, titanium, vanadium, aluminum, or nitride.
Embodiments of the present invention decrease current density and current crowding on the top metal layer as well as on the metal layer immediately below the top metal layer of the die. For example, for a die having 8 metal layers, when current crowding exceeds EM (electromigration) margins on the 7th metal layer, it is desirable to use multiple passivation openings to connect the C4 bump to multiple metal lines on the top (8th) metal layer. Where multiple passivation openings are used to connect a single C4 bump to multiple metal lines on the top metal layer, current crowding may be decreased by up to 90%. The higher power regions of the die may also be defined as those regions for which a decreased IR drop is desirable. Embodiments of the present invention allow a decrease in IR drop for regions of the die where multiple passivation openings are used to connect the C4 bump to multiple metal lines on the top metal layer. Simulations show as much as a 60% improvement in IR drop. This improvement is cause by a reduction in resistance for the top two metal layers due to the enlarged passivation opening(s).
In one embodiment, C4 bumps 503 located in the high power region of the die are each connected to two narrow metal lines 508A or 508B on the top metal layer by two passivation openings 510. In one particular embodiment of the present invention, each passivation opening 510 which connects a single C4 bump to two narrow metal lines may be approximately 6 um×30 um in size, or a total area of approximately 360 um2. Thus, in the high power region of the die, embodiments of the present invention may be used to increase the total area of the connection between the C4 bump and the die, as compared to the area of the connection in the low power region.
Power (Vcc) and ground (Vss) in the high power region 520 of the die are distributed to the die on alternate pairs of narrow top metal lines 508A and 508B. Narrow metal lines 508A are power rails (Vcc), and narrow metal lines 504B are ground rails (Vss). In the high power region, narrow metal lines 508A and 508B run substantially parallel to one another, and are approximately 10 um wide. The spacing between the narrow metal lines 508A and between narrow metal lines 508B may be between 20 and 70 um, and more particularly may be approximately equal to 50 um. Metal lines 508 distribute power to the lower metal layers, including metal lines 512. Metal lines 512 are on the top-1 metal layer lie directly beneath the top metal layer and run substantially perpendicular to top layer metal lines 508. In the high power regions of the die, power must travel a distance equal to DH on the top-1 metal lines 512 between the top layer Vcc and Vss C4 bumps to complete the current path. In one embodiment, this distance may be approximately 114 um. Thus, in the high power region, where multiple passivation openings are used under each C4 bump power travels a distance on the top-1 metal lines 30% shorter than that which may be achieved using a single passivation opening. Because the distance power must travel on the top-1 metal layer may be decreased significantly by the use of multiple passivation openings to connect a C4 bump to multiple metal lines, both the IR (voltage) drop and current crowding are reduced on the top-1 metal layer. Additionally, when using a multiple passivation opening topology, the width of the top-1 metal lines 512 may be decreased by approximately 30% to effectively route power to the die. This allows a greater number of signals to be routed on the top-1 metal layer.
In one embodiment, die 500 may also include a low power region. Low power regions are those regions where it has been determined that the EM margins and IR drop are adequate, and thus the EM margins and IR drop do not require reduction. In regions of low power, a single passivation opening 506 may be used to connect each C4 bump 502 to a single metal line, 504A or 504B, on the top metal layer of the die for power and ground connections. Single passivation openings may also be used for C4 bumps which are connected to I/O signals on the die. In one embodiment of the present invention, each passivation opening 506 which connects a single C4 bump to a single wide metal line may be approximately 16 um×16 um in size, or approximately 256 um2. Power and ground in the low power region 510 of the die are distributed to the die on alternating wide top metal lines 504A and 504B. Wide metal lines 504A are power rails (Vcc), and wide metal line 504B is a ground rail (Vss). In the low power regions of the die, power must travel a distance equal to DL on the top-1 metal lines 512 between the top layer Vcc and Vss C4 bumps. In one embodiment, this distance may be approximately 160–170 um. In the low power region, metal lines 504A and 504B may run substantially parallel to one another, and may be approximately 20 um wide. The spacing of the wide metal lines 504 may be approximately 70–75 um. Metal lines 504 distribute power to the lower metal layers, including metal lines 512. Metal lines 512 on the top-1 metal layer lie directly beneath the top metal layer and run substantially perpendicular to top layer metal lines 504.
Multiple passivation openings 610 may be used to connect each C4 bump 603 to multiple metal lines 608 on the top metal layer of the die for power and ground connections in the high power regions of the die, as described above with respect to
Next, as set forth in block 720, passivation openings may be formed in the passivation layer. The passivation openings may be formed by patterning using a conventional lithography process. The passivation openings may be any shape or size, so long as they are no larger than the C4 bump that covers the passivation opening. In embodiments, the passivation openings may be square, rectangular, octagonal, or circular in shape. The passivation openings may range in size from approximately 50 um2 to 500 um2. When formed, the passivation openings may expose metal lines on the top metal layer of the die.
After the passivation openings are formed, a plurality of C4 bumps may be placed on the top surface of the die, over the passivation layer, as set forth in block 730. In the areas of high power, as described above, each C4 bump may cover at least two passivation openings, and may connect to at least two top layer metal lines. In areas of low power, or for I/O signals, each C4 bump may cover only one passivation opening, and may connect to only one top layer metal line.
Finally, as set forth in block 740, a package may be placed over the die and the C4 bumps, so that the C4 bumps electrically connect the die to the package.
The present invention may be implemented with various changes and substitutions to the illustrated embodiments. For example, as semiconductor technology scales to even smaller dimensions, the dimensions mentioned herein may be scaled down as well. Although specific embodiments, including specific parameters, methods, and materials have been described, it will be readily understood by those skilled in the art and having the benefit of this disclosure, that various other changes in the details, materials, and arrangements of the materials and steps which have been described and illustrated in order to explain the nature of this invention may be made without departing from the principles and scope of this invention as expressed in the subjoined claims.